gistfile1.txt

class Order < ActiveRecord::Base
  has_many :order_items, :inverse_of => :order
  accepts_nested_attributes_for :order_items
end

class OrderItem < ActiveRecord::Base
  belongs_to :order, :inverse_of => :order_items
  accepts_nested_attributes_for :order
end

> order = Order.new
 => #<Order id: nil, created_at: nil, updated_at: nil> 
> order_item = OrderItem.new
 => #<OrderItem id: nil, order_id: nil, created_at: nil, updated_at: nil> 
> order.order_items << order_item
 => [#<OrderItem id: nil, order_id: nil, created_at: nil, updated_at: nil>] 
> order.valid?
SystemStackError: stack level too deep